Footwear, in particular athletic footwear, are expected to provide proper shock absorption and stability thereby preventing potential harmful effects of vigorous movements such as running and jumping on the wear's feet. The footwear industry has been developing athletic shoes in an effort to maximize shock absorption and stability while also maximizing comfort and durability. Unfortunately, these goals are potentially in conflict with each other. For example, a shoe that provides adequate shock absorption and comfort may not provide sufficient stability. To further advance the development of athletic shoes, a basic understanding of the dynamics of running and the mechanisms of running injuries is important.
A typical walking or running gait cycle involves two phases: (1) a stance phase, and (2) a swing phase. One foot contacts the support surface such as the ground and bears weight in the stance phase while the other foot is moving through the air and advances in the swing phase. The two phases are repetitive. The difference between the running and walking gait cycles is that at one point during the running cycle the person is airborne without bearing any weight, whereas the walking cycle does not have such an airborne point.
The stance phase of a running gait cycle may be further divided into three periods: (1) the loading period, also called the impact and support period or the heel strike period, (2) the mid-stance period, also called the mid-stance and propulsion period, and (3) the toe-off period, also called the recovery period. For a typical runner of a heel-to-toe running style, the loading period begins with first contact of the heel with the running surface, followed by a controlled lowering of the forefoot to the running surface. The first contact of the heel typically occurs at the rear, outer part of the heel. The mid-stance period begins once the forefoot is in contact with the running surface. During the mid-stance period, the contraction of the musculature of the leg generates power to propel the body forward. The heel progressively lifts and the forefoot flexes at the metatarsophalangeal joint. Then in the toe-off period, the foot disengages contact with the running surface and the foot becomes airborne.
Pronation is a normal movement of the foot that occurs during the loading and mid-stance periods of the stance phase of the gait cycle. At heel strike during the loading period, the heel of the foot is supinated and makes initial contact with the running surface as described earlier. Instantaneously, the joint between the foot bones called the subtalar joint is unlocked, allowing pronation, a coordinated triplane motion of the foot, to occur during the forefoot lowering events of the loading period of the stance phase. The coordinated triplane motion of the foot involves three planes of motion: (1) abduction, in which the front of the foot is turned outwards and away from the line of progression of the runner; (2) dorsiflexion, in which the front of the foot is angled upwards relative to the heel of the foot; and (3) eversion, in which the sole of the foot is turned outward relative to the heel of the foot. With the combination of these three motions, the foot rolls from the outside or lateral side to the inside or medial side of the foot resulting in the medial aspect (the arch area) of the foot coming into contact with the running surface, thus allowing the foot to adapt to the running surface and to transfer some of the loading force to the running surface, thereby reducing the risk of injury during the stance phase of running. The pronated position of the foot is maintained throughout the mid-stance period.
Supination typically follows pronation. As the body moves forward over the foot, the subtalar joint locks. This allows a reversal of the events that have occurred during the loading period to occur during the mid-stance period. Supination is a coordinated triplane motion of the foot, which involves three planes of motion: (1) adduction, in which the locking of the subtalar joint allows the foot to turn inward toward the line of progression; (2) plantarflex, in which the forefoot is flexed downward relative to the heel; and (3) inversion, in which the sole of the foot is turned inward relative to the heel. With the combination of these three motions, the foot continues rolling forward onto the toes. During motion through ball and toe contact, the foot rolls outward just before the toes starts to leave the ground. The combination of these motions allows the foot to be converted from a mobile adaptor to a rigid lever, which is essential for the forward propulsion of the body. The foot remains supinated while it is off the ground between steps.
Although pronation is a natural action and is considered an important and healthy response to the intense amount of shock imposed upon the foot, excessive pronation and high pronation velocity have been suggested by biomechanists to cause a variety of injuries at the ankle, knee and hip among runners and other athletes. Many prior art soles have been designed to control pronation and supination. However, as the stability of the sole increases to control the amount of lateral motion of a foot in order to prevent excessive pronation, the shock absorption properties for reducing the impact of strike forces on the foot usually decrease. Thus, the footwear industry continues to seek a proper balance between the stability and shock absorption properties in designing shoe soles.
For Example, U.S. Pat. No. 5,625,964, issued to Lyden et al., discloses an athletic shoe having a sole with a rearfoot strike zone segmented from the remaining heel area by a line of flexion which permits articulation of the strike zone during initial heel strike of a runner. The line of flexion is located to delimit a rearfoot strike zone reflecting the heel to toe running style of the majority of the running population. In addition to allowing articulation of the rearfoot strike zone about the line of flexion, the sole incorporates cushioning elements, including a resilient gas filled bladder, to provide differential cushioning characteristics in different parts of the heel, to attenuate force applications and shock associated with heel strike, without degrading footwear stability during subsequent phases of the running cycle. The line of flexion may be formed by various ways including a deep groove, a line of relatively flexible midsole material, and a relatively flexible portion of a segmented fluid bladder.
The athletic shoes presently available on the market are typically of a multiple layer construction comprised of an outsole, a midsole and an insole. The outsole is normally formed of an abrasion-resistant material such as rubber and is the portion of the sole that contacts the ground. The midsole is the portion between the outsole and the insole and is typically comprised of a compressible material such as ethylene vinyl acetate (EVA) foam for cushioning. The insole is the portion in contact with the wearer's foot and is normally comprised of a soft pad to enhance shoe comfort.
Durability of the midsole is also an important goal for sole design. Foam materials such as the EVA foam commonly used in the midsole have limited useful lives and tend to break down over time. Alternative midsole designs that are not or less dependent on the foam materials have been developed over the past years.
For example, U.S. Pat. Nos. 4,536,974, 4,611,412, 4,754,559 and 4,573,021, all issued to Eli Cohen, describe midsoles provided with a plurality of pairs of ribs. All of the ribs are provided with at least one bowed or convex surface running the length of the rib. When weight is placed upon the sole, each of the ribs initially begins to deflect until adjacent ribs abut one another at which point the ribs begin to compress. Inserts may be placed between adjacent rib pairs to fill the space between the adjacent rib pairs to inhibit the deflection of the ribs. Compressible bridging elements may be provided between the pairs of ribs to avoid the noises resulted from the constant contact and releasing of the ribs of adjacent pairs.
U.S. Pat. Nos. 5,461,800 and 5,822,886, both issued to Simon Luthi et al., describe integrally molded midsoles having tubular suspension members. The tubular suspension members behave as springs and have spring constants which may be designed for a particular application by choice of the tube length, the tube wall thickness or the hardness of the tube material. Preferably, the midsole is made of an elastomer such as HYTREL® that is cast in a preformed shape and thereafter subjected to substantial compressive forces so that the tubular springs take a compression set and thereafter perform as near-ideal springs.
U.S. Pat. No. 5,337,492, issued to Wolf Anderié et al., describes a shoe bottom having a plurality of individual flexurally resilient carrier elements which are directed transversely with respect to the longitudinal direction of the shoe and which are arranged at spacings one behind the other in the longitudinal direction of the shoe. The carrier elements are connected to a cover plate portion on the foot side and to an outsole layer on the outward side. Each carrier element is formed by a closed box profile with an upper web portion which extends transversely with respect to the longitudinal direction of the shoe, a lower web portion which is parallel to the upper web portion, two lateral support walls which connect the ends of the web portions together and bracing means supporting the upper web portion relative to the lower web portion.
U.S. Pat. No. 6,769,202, issued to Simon Luthi et al., describes a sole unit for a shoe including a directional element, a cushioning element and, optionally a heel cradle. The directional element has a top plate, a bottom plate and multiple generally parallel strut elements oriented transversely to the longitudinal axis of the directional element and connected to the top plate and the bottom plate. The cushioning element is adapted to be received in the directional element, more specifically between the strut members of the directional element.
The prior art soles described above do not provide the shoes with optimal shock absorption and stability due to their design. The present invention seeks to provide a midsole for a shoe which provides superior shock absorption and stability properties and which can be customized for different applications and individuals.